A wide variety of polymeric films are produced using extrusion technology. The composition of the film is dependant upon the end use to which the film is put. Examples of polymeric materials suitable for processing in an extrusion system for the production of film are polyethylene (high density, low density and linear low density), EVA, EVOH, polyamides, etc. Such materials can be processed in an extrusion system for blown film of single layer and co-extruded films with up to seven or more layers, including barrier and tie-layers. Typically, blown films or sheet films have thicknesses in the range of 0.4 to 40 mils. These films are useful in the food packaging and other packaging industries, as well as agricultural, automotive and a wide variety of other industries.
There are two principle types of extruders for melting and extruding polymeric materials in film production systems. The first is a "smooth bore" or smooth barrel extruder which has a smooth barrel over its entire length. There are numerous North American manufacturers of smooth bore extruders, including Davis Standard, Gloucester Engineering, Cincinnati Milacron and Brampton Engineering, among others.
In the production of polymeric film using a smooth bore extruder, generally the polymeric resin raw material is fed to the extruder in the form of pellets. Because waste and scrap film are generated during production of the polymeric film, which material can be recycled within the production facility, it is not uncommon to blend some proportion of the recycled material with the pellets. The scrap film is chopped using techniques and equipment well known in the art and is often referred to as "fluff" or "flake". This fluff can be fed into a smooth bore extruder along with the "virgin" resin pellets at ratios up to 50/50. If the equipment is specially designed, as much as 100% fluff can be fed into a smooth bore extruder. This is typically accomplished by a speed-controlled (ratio controlled) auger located in a hopper above the screw of the extruder. With this technique, there is no pre-mixing of the fluff with the virgin pellets and there is no control of the "head pressure" on the extruder throat. Another method for feeding fluff into a smooth bore extruder is to mix the fluff and pellets together and then introduce the blend into the throat of the extruder. This blend is force fed into the extruder using a specific type of feeder equipment commonly referred to as a "crammer" feeder. The force exerted by the crammer feeder pushes the blend into the extruder and is principally controlled by a torque setting. The speed of the crammer feeder auger is then adjusted to maintain the torque setting and thus the "head pressure" on the extruder screw. Because a smooth bore extruder's feed/pump rate output is significantly affected by the head pressure at the inlet, the second method for feeding fluff may be considered superior to the first because of the relative constancy of the head pressure.
The second common type of extruder is called a grooved feed extruder. Although used worldwide, these are commonly built in Germany and are available from companies such as Hosokawa Alpine Aktiengesellschaft of Augsburg, Germany. Such extruders were initially developed for processing high molecular weight, high density, polyethylene (HMWHDPE). Subsequently it was found that the pumping and melting characteristics of such extruders had certain advantages in the processing of other polymers also, particularly in blown film production applications. A grooved feed extruder has longitudinal grooves formed in the barrel beginning just downstream from the barrel inlet. These grooves do not extend the length of the barrel. As is well known in the art, the grooves are highly efficient at transferring energy from the extruder motor to the polymer and cause the polymer to rapidly melt very close to the extruder inlet.
Conventional wisdom has suggested that making extruded film in a grooved feed extruder utilizing fluff in the raw material, in any percentage, is not possible or not likely to be successful. It has been known to run in the range of 5-10% fluff into a grooved feed extruder via a typical gravity feed-type hopper. However, because of the perceived problems and problems actually experienced in feeding fluff to grooved feed extruders, manufacturing facilities that use grooved feed extruders typically send their scrap film (fluff) through an additional process step of re-pelletization and simply recycle and reuse the scrap in pellet form. It was and is believed that the reasons fluff cannot successfully be processed through a grooved feed extruder are: (1) that the fluff melts in the grooves and either plugs them or carbonizes or forms gels, which then may break loose and appear in the extruded film, thus rendering it non-usable; and (2) that fluff cannot readily and efficiently be fed to the grooved bore extruder and thus extruder output and capacity are diminished, thereby resulting in increased film cost.
What is needed is an efficient, enhanced output system and method for production of polymeric film in which productivity from the extruder is increased and a system and method are capable of utilizing anywhere from 0%-100% fluff in the feed material for the extruder.